1. Field of the Invention
The present invention relates generally to plasma processing systems and methods, and in particular relates to inductively coupled high-density plasma sources suitable for use in a plasma processing system.
2. Background of the Invention
Ionized gas or “plasma” may be used during processing and fabrication of semiconductor devices, flat panel displays and other products requiring etching or deposition (“plasma processing”) of materials. Plasma may be used to etch or remove material from semiconductor integrated circuit wafers, or sputter or deposit material onto a semiconducting, conducting or insulating surface. Creating a plasma for use in manufacturing or fabrication processes typically is done by introducing a low-pressure process gas into a chamber surrounding a workpiece, such as an integrated circuit (IC) wafer, that resides on a workpiece support member, more commonly referred to as a “chuck.” The molecules of the low-pressure gas in the chamber are ionized into a plasma by a plasma source after the gas molecules enter the chamber. The plasma then flows over and interacts with the workpiece, which may be biased by providing RF power to the chuck supporting the workpiece.
To be most effective in plasma processing, the plasma preferably has a high-density (measured as the number of electrons or ions per cubic centimeter) and is uniform. High-density plasma processing can increase throughput and therefore increase production in semiconductor manufacturing. Furthermore, the plasma preferably has a small volume (thin and flat) so that the radicals in the process system have a short residence time. A short radical residence time permits control of the proper dissociation of radicals in the plasma volume for achieving high rate, selective etch in high aspect ratio etch features.
One type of plasma source that has been developed and commonly used is a parallel-plate, capacitively coupled plasma (CCP) source. Such a source uses radio-frequency (RF) power sources to generate the plasma through gas discharge. These power sources typically operate at 13.56 MHz, but can operate at other frequencies. Parallel-plate plasma sources usually have small gap spacing and small plasma volume. However, they typically generate low-density plasmas of less than 1011 ions/cc which limits the etch rate.
Another type of plasma source is an electron cyclotron resonance (“ECR”) source, which uses microwave (2.45 GHz) energy sources to generate a plasma having relatively high densities, on the order of 1011-1012 ions/cc and greater. Although an ECR source provides a relatively high plasma density and good control of ion energy, it requires in the plasma source a significant magnetic field, which is normally undesirable in the processing reactor. In addition, difficulties arise in generating uniform plasmas over large wafer areas.
A third type of plasma source is an inductively coupled plasma (ICP) source, which uses an inductively coupled radio-frequency power to generate the plasma This type of plasma source provides for a relatively high plasma density (1012 ions/cc or greater) and operates with a radio-frequency source (typically 13.56 MHz). However, a shortcoming of conventional inductively coupled plasma sources is a non-uniform plasma density in the region above the substrate. The plasma volume is also very large, resulting in very long residence times for the radicals, which limits the etch rate.
A fourth type of plasma source is the Helicon plasma source, which uses a relatively constant volume magnetic field. It is capable of generating a very high density (1013 ions/cc) and operates with a radio-frequency source (typically 2-30 MHz). The Helicon source requires propagation and damping of the low frequency whistler wave in a system with minimum length greater then one half of the propagating wavelength. In short systems, the plasma generating efficiency is usually reduced drastically.
For those prior art systems capable of producing a high-density plasma (in excess of 1012 ions/cc), efficiency is generally sacrificed in producing plasma in a small volume. This inefficiency makes high-density plasma processing a costly proposition for manufacturing purposes.